1. Field of the Invention
The invention relates to a radio communication apparatus which is applicable to a mobile station in a digital mobile radio communication system.
2. Description of the Prior Art
FIG. 4 shows a block diagram of a conventional radio communication apparatus which is applied to a mobile station in a digital mobile radio communication system. In FIG. 4, an analog output voice signal from a microphone 1 is converted to a digital signal in a linear codec 3. A digital signal processor (DSP) 4 executes a coding process, a data compression process and an error correction process according to the voice coding procedure stored in a ROM 50 and outputs a coded data to a radio modem 6. The radio modem 6 modulates the coding data received from DSP 4 according to the indication from a control circuit 90, which controls the devices in the radio communication apparatus, and transmits the modulated radio signal to the base station (not shown) via an antenna 7.
In the case of receiving the radio signal from the base station, the input radio signal is input to the radio modem 6 via the antenna 7. The radio modem 6 demodulates the modulated signal and sends the demodulated signal to the DSP 4. The DSP 4 decodes the coded signal according to the voice decoding procedure stored in the a ROM 50. The control circuit 4 sends the decoded digital signal to the linear codec 3. The linear codec 3 converts the decoded digital signal to an analog signal and send it to the speaker 2. The speaker 2 converts the analog signal to a voice output.
A line quality monitor 8 measures the line quality of the received signal via radio modem 6 and sends the information regarding to the line quality to the control circuit 90. The control circuit 90 selects the signal coding/decoding procedures and other related parameters.
An example of the voice coding/decoding procedure is the Vector Sum Exited Linear Prediction (VSELP) system which is adapted in the U.S.A. digital cellular system according to the IS-54 regulation of Telecommunication Industry Association (TIS). In the above system, the code bit rate of the code data is 8 K bits per sec (Kbs) for voice coding and 5 Kbs for coding of the error correcting. Therefore the systems according to the regulation can maintain the voice quality above a certain standard level even if the line quality is very poor. But, in the above system, the error correcting code bit occupies a very large number of bits compared to the voice code bit.
As discussed above, this system is designed to be applied in a very poor line quality condition. Therefore, if the line quality is good, the systems has over-correction bits for the transmission signal. Accordingly, the system incorporates the excessive correcting capability and the DSP 4 consumes a large amount of power.
It is a primary object of the present invention to provide a radio communication apparatus which selects a suitable procedure according to the line quality.
It is further object of the present invention is to provide a radio communication apparatus which reduces the consumed power in the case of good line quality.